1. Field of the Invention
The present invention relates to a ZnO system semiconductor device comprising a p-type layer formed of a p-type ZnO thin film, doped with nitrogen atoms as a p-type layer of a PN junction in a semiconductor device, such that the thin film growth direction is conformed to the direction of Zn polarity (0001).
2. Description of the Related Art
A ZnO (zinc oxide) thin film is optically transparent and has a piezo-electric property and characteristics of a n-type semiconductor, and is also useful as materials, for example, such as those for a surface-acoustic-wave (SAW) device and a transparent thin film transistor (TPT). Among those, many devices have already been put to practical use in the form of polycrystal, such as a surface acoustic wave device, a gas sensor, a piezoelectric device, and a pyroelectric device.
Moreover, the ZnO thin film is expected to become applicable to photoelectron devices as well, in addition to these applications. To be more specific, ZnO constitutes a direct transit semiconductor wherein the wave number of the minimum value of the conduction band is equal to the wave number of the maximum of the valence band, with a large forbidden-band width (energy gap is about 3.4 eV), so that the ZnO has been expected as a materials for photoelectron devices applicable to a band ranging from the blue to ultra-violet. Even as compared with GaN system compound covering almost the same wavelength band, excited electrons and holes have the same wave number, and easily emit light by recombination without being accompanying by any change in kinetic momentum for the recombination, resulting in its remarkably high exciton binding energy (60 meV for ZnO, and 24 meV for GaN). Accordingly, ZnO has attracted attention since photons excellent in monochromaticity can be released by utilizing a highly efficient exciton luminescence process.
Thus, there are expected also applications as light emitting-and light receiving devices in the blue and ultra-violet band by utilizing its characteristic of wide band gap semiconductors. On the other hand, there are known in case of the ZnO thin film crystal, defects and etc. due to oxygen holes and interstitial zinc atoms. On occurrence of such crystal defects, electrons not contributing to atomic bonding are generated and thus ZnO film exhibits a n-type conductivity. In order to carry out conduction-type control by impurities, reduction of these defects is essentially important whereby p-type doping becomes possible by reducing the concentration of such residual electrons.
However, since difficulties are caused by the growth of high quality thin film material as well as the reduction of the concentration of residual electrons probably due to the oxygen holes, p-type has been difficult to fabricate.
Under such a situation, there has been made an invention for the purpose of obtaining a semiconductor light emitting device for a wide range of light from green to ultra violet, employing ZnO system materials of hexagonal system so as to take into account environmental circumstances (refer to patent 1 in related art document below), but there has been few disclosures about p-type ZnO material serviceable in practical use, with the result that there has been posed a problem involved in the development of ZnO system semiconductor devices causing no degradation in luminous efficiency.
In terms of the fabrication of p-type ZnO thin films, there is a report that p-type ZnO thin films, with nitrogen atoms doped, have been formed by using a Zn-polar ZnO bulk substrate (refer to non-patent document 1 in related art document below).
Moreover, since there is required a film formation method of high quality ZnO single crystal thin films, enabling to prevent the generation of crystal defects, controlling the polarity of ZnO is under investigation. Recently, there is a report that it is possible to control the film formation direction of a ZnO thin film so as to achieve O polarity or Zn polarity, in such a manner as to grow ZnO on a Ga-polar GaN substrate, with a polarity controlled (refer to non-patent document 2 in related art document below).